binary_search(array, key)
   n = length of array
   low = 0
   high = n - 1
   mid = (low + high) / 2
   while (array[mid] doesn't match key)
       if (array[mid] > key)
           high = mid - 1
       else
           low = mid + 1
       if (low > high)
           return no match
       mid = (low + high) / 2
   return array[mid]